Retrotransposons are ubiquitous features of mammalian genomes, constituting about 15 % of the total DNA. They are known to cause mutations by insertional inactivation of genes, by changing the regulation of gene expressions, and by acting as sites for recombination events which lead to chromatin rearrangements. In order to better understand the ways in which transposable elements affect the organisms in which they occur, we need a more accurate assessment of transposition rates, and an understanding of the mechanisms by which elements within the genome interact. The rate of transposition in mammals has not been directly measured, in part because of the large number of elements in the genome. However, the age distribution of elements within the genome can be assessed by sequence analysis. The age distribution is the sum of all activities of the elements, including transposition, removal, and gene conversion. If the magnitude of these factors can be determined, the age distribution of elements in the genome can be used to estimate transposition rates. It is not known if there is a mechanism which removes retrotransposons from the mammalian genome. One line of evidence which suggests that removal of elements may occur during recombination is the preferential accumulation of some types of elements on the X chromosome. If elements are removed during recombination, this would occur in both sexes from the autosomes, but only in the female from the X chromosome; thus removal would occur more slowly from the X. The first objective of this proposal is to use the age distribution of retrotransposons on the X chromosome versus the autosomes to test this hypothesis, and the alternate hypothesis that accumulation on the X is due to a higher rate of insertion in the X than in the autosomes. It is not known to what extent elements within the genome undergo gene conversion. The second objective of the proposal is to examine elements at the same site in related species to determine if elements within the genome interact to exchange sequence information. The third objective is to examine the rate of transposition of two transposable elements in the same genome to see if the rates of transposition are correlated.